1. Field of the Invention
This invention relates to a method of forming silicon dioxide and more particularly, to a method including reacting dichlorosilane with a gaseous oxidant to form silicon dioxide.
2. Description of the Prior Art
Waveguides used in optical communications systems referred to as "optical waveguides," are normally constructed from a transparent dielectric material such as glass or plastic.
It is well known to one skilled in the art that light can be caused to propagate along a transparent fiber structure which has a higher refractive index than its surroundings. The ordinary use of such optical fibers is to transmit a signal or an image, that is light which has been modulated from one point to another. Optical fibers produced for these purposes must avoid excessive attenuation of the transmitted light to be effective. Also, to be an effective transmitting medium for an optical communications system, an optical waveguide should not only transmit light without excessive attenuation, but should be constructed to minimize cross-talk from adjacent waveguides. In addition, such an optical waveguide should not cause dispersion of the transmitted light and should propagate preselected modes of light.
Producing a satisfactory optical waveguide has been one of the more difficult problems in the development of an effective optical communications system.
A method heretofore used for producing an optical fiber is described as follows: A silica tube is selected and maintained at an elevated temperature of about 1100.degree.-1200.degree. C. Dry oxygen is bubbled through a tank containing liquid silicon tetrachloride, SiCl.sub.4, at a temperature of approximately 35.degree. C. SiCl.sub.4 vapors picked up by the oxygen are then directed through the silica tube where at the inside wall there is a reaction between the oxygen and the SiCl.sub.4 resulting in the formation of very pure SiO.sub.2 which is deposited on the inside wall of the silica tube in the form of a first SiO.sub.2 layer. A second layer comprising SiO.sub.2 suitably doped, e.g,, with titanium, is deposited on the first layer employing chemical vapor deposition as described except the oxygen is bubbled through a mixture of SiCl.sub.4 and a dopant, e.g., TiCl.sub.4. The resultant laminar structure is then drawn until the tube collapses and is fused together to form an optical fiber preform. In this method, the reaction of SiCl.sub.4 and O.sub.2 (as well as with H.sub.2 O, N.sub.2 O, CO.sub.2 and other gaseous oxidants) is very temperature dependent and variations in reaction temperature lead to irregularities in the thickness and in the uniformity of the resultant SiO.sub.2 layers.
U.S. Pat. No. 3,459,673 reveals a method of producing a doped luminescent silica glass where silica is formed in a process including reacting a hydrolyzable silicon compound, such as SiH.sub.2 Cl.sub.2, with water. U.S. Pat. No. 3,459,673 however does not reveal a method of depositing a uniform SiO.sub.2 layer on a substrate surface at a rate which is temperature independent and therefore relatively unaffected by temperature gradients.
Accordingly, a method of forming a relatively defect free SiO.sub.2 preform which is relatively rapid and which can be carried out at wide temperature ranges including relatively lower temperatures and which is relatively temperature insensitive is an object of the subject invention.
In the manufacturing of planar semiconductor devices, the entire surface of the wafer or body of the semiconductor material is coated with an insulating or masking layer, which is normally a layer of silicon dioxide. By means of photolithographic techniques including coating the surface of the insulating layer with a uniform layer of photoresist, exposing the same to a desired pattern on a photolightographic mask, and subsequent chemical etching, selected portions of the insulating layer are removed to expose the surface of the semiconductor body. A desired P-N junction may then be formed in the semiconductor body by ion implantation techniques or by predepositing the desired impurity material on the exposed surface of the body and then diffusing the impurity into the body by well known solid state diffusion techniques. The diffusion of the impurity material is normally carried out under such conditions that a layer of oxide is regrown over the previously exposed surface of the semiconductor body resulting in a continuous oxide layer across the entire surface. Additional junctions, for example in the case of double diffused transistor devices, may be formed in the wafer by selectively removing a portion of the regrown oxide layer by photolithographic techniques to expose the surface of the semiconductor body and then predepositing and diffusing the desired impurity into the body again under conditions whereby the oxide layer is regrown over the exposed portion. Electrical contacts to the various P-N junctions are then provided by once again selectively removing portions of the oxide to expose the surface of the wafer thereunder, evaporating a metal film over the entire surface of the oxide and selectively removing portions of the film by photoengraving techniques.
The masking silicon dioxide layer has been thermally grown by oxidation of silicon which is normally performed at relatively high temperatures, above 900.degree. C and usually from 1100.degree. C to 1200.degree. C. Thermally forming the oxide at such temperatures, i.e., above 900.degree. C, can cause a shift of the P-N junction boundaries during the oxidation of silicon as well as creating dislocation and stacking faults within the semiconductor body itself. Thermally grown oxides resulting from temperatures below 900.degree. C are formed at a very low rate and therefore require long periods of time thereby making such a process uneconomical for semiconductor device manufacture.
Other techniques for forming silicon dioxide films include gaseous oxidation of SiCl.sub.4 or SiH.sub.4. These techniques involve undesirably high temperatures, e.g., around 1100.degree.-1200.degree. C, which again can cause a shift of the P-N junction boundaries during their formation as well as creating dislocation and stacking faults within the silicon body itself. Low temperature (below 900.degree. C) gaseous oxidation of SiH.sub.4 or SiCl.sub.4 results in silicon oxide films which are very porous and which can easily be dissolved or etched. Furthermore, the deposition rate with silica is very slow. For planar semiconductor device fabrication, a dense silicon oxide film is desired as well as one which has a low dissolution or etch rate.
A relatively low temperature technique of forming a uniform, relatively non-porous silicon dioxide film on a surface of a semiconductor body which does not introduce additional defects into the semiconductor body is desired.